There are available to the clinical analyst various test devices for the rapid analysis of certain constituents in blood samples which if found to be in excess of a predetermined concentration indicate a potential pathological condition. Such tests include means for detecting glucose and other sugars such as galactose. Other constituents normally present in blood samples include higher molecular weight materials such as albumin, cholesterol and triglycerides.
Many of the tests and methods for blood analysis have encountered difficulties when whole blood is analyzed because the test devices utilize chromogenic or other visual responses to indicate the presence or absence of the constituents being sought and the presence of red coloration in whole blood interferes with these colormetric determinations. In addition, for analytes other than hemoglobin, the concentration within the erythrocyte may be different from the plasma concentration. Accordingly, rupture of the erythrocytes may introduce undesirable changes into the assay results. This can be problematical in assay techniques which employ responses other than the formation of chromogens, such as those involving precipitation and enzyme catalyzed reactions.
Various techniques have been developed for separating erythrocytes from whole blood samples to provide blood plasma which can be analyzed without their interference. These methods typically involve precipitation and filtration in which various means for enhancing the separation are employed. One such means is disclosed in U.S. Pat. No. 3,552,925 in which there is described contacting a whole blood sample with a water soluble salt containing an inorganic cation to thereby enhance the separation of erythrocytes from the liquid fraction. The salts disclosed are primarily those having mono- or divalent cations although in one instance ferric chloride at a concentration of 1 molar is mentioned. In all cases, the salt is employed at a concentration in the range of from 1/4 molar to saturation except in one embodiment of the system disclosed in which pieces of filter paper are immersed with a solution containing from 1 to 60% by weight of the salt and subsequently dried to leave a salt residue. This system works well when the blood is to be analyzed for low molecular weight highly soluble analytes such as glucose, however, higher molecular weight analytes such as cholesterol are separated along with the erythrocytes leaving a plasma sample which is unsuitable for the analytical determination of these high molecular weight analytes.
In German published patent application 34 41 149 there is disclosed a method for separating erythrocytes from diluted blood by the use of lectin. This process involves placing the blood sample on a matrix impregnated with lectin and separating the erythrocytes by thorough rinsing with a diluent.
It would be desirable and it is an object of this invention to provide a method for the separation of erythrocytes from whole blood using dilute salt solutions which will agglutinate erythrocytes without the concomitant precipitation of high molecular weight analytes.